1. Technical Field of the Invention
The present invention relates to current mirror circuits, and particularly to current mirrors having reduced nonlinear distortion.
2. Description of the Related Art
Current mirrors are widely used in analog integrated circuits. In general terms, current mirrors are circuits having a reference branch through which a reference current flows and at least one mirror branch through which a current flows that is proportional to the reference current flowing through the reference branch.
Efforts to improve the performance of current mirrors resulted in the creation of a wide variety of different implementations. A relatively popular current mirror implementation is the cascoded current mirror shown in FIG. 1. Reference branch 10 includes transistors 11 and 12 coupled together in cascode relation. Mirror branch 13 includes cascode connected transistors 14 and 15. The control terminals of transistors 11 and 14 are connected to a reference or bias voltage Vbias. The control terminal of transistors 12 and 15 are connected to each other and to the input of the current mirror circuit. The output current of the current mirror, Iout, passes through mirror branch 13 and is proportional to the current Iin passing through reference branch 10. The relationship between the output current Iout and input current Iin is based upon the ratio of the sizes of transistors 14 and 15 to the sizes of transistors 11 and 12. At relatively low frequencies, the current mirror of FIG. 1 exhibits relatively accurate proportionality and relatively low nonlinear distortion.
At high frequencies, however, parasitic capacitances 16 in the current mirror of FIG. 1 adversely affect the amount of nonlinear distortion. Because the transconductance of a MOS transistor is inherently nonlinear, the voltage appearing at the input of the current mirror of FIG. 1 is not linearly proportional to the input current Iin and is subject to nonlinear distortions. This may be seen with reference to FIG. 2, in which the input current and input voltage of current mirror 10 is shown. Excursions of the input voltage are greater when the input current is low. The typically high output impedance of a current source (providing input current Iin to current mirror 10) and the reference current branch do not allow the input current to be affected by the nonlinearity of the input voltage at low frequencies wherein the current flowing through parasitic capacitor 16 is negligible. At high frequencies, the current passing through parasitic capacitance 16 becomes comparable with the input current Iin. Parasitic capacitance 16 is formed from the output capacitance of the current source providing input current Iin to the current mirror, the drain capacitance of transistor 11 and the gate capacitances of transistors 12 and 15. The current passing through parasitic capacitance 16 at high frequencies adversely affects the transfer function of the current mirror. In addition, because the charge accumulated at parasitic capacitance 16 is substantially proportional to the input voltage and because the input voltage is nonlinear relative to the input current Iin, additional nonlinear distortion to the input current Iin is exhibited. These distortions are transferred to the output current Iout in mirror branch 13.
An attempt to improve the nonlinear distortion in the current mirror of FIG. 1 is shown in FIG. 3. A resistor 17 is connected to the source terminal of each transistor 12 and 15. Resistors 17 tend to make more linear the effective transconductance of the reference branch 10 and mirror branch 13 at lower frequencies. However, the effect provided by resistors 17 dwindles at higher frequencies. A transistor 18 is connected between the input of the current mirror and the gate terminals of transistors 12 and 15 so as to decouple the input capacitances of transistors 12 and 15 from the input. Though transistor 18 reduces nonlinear distortions at higher frequencies, transistor 18 reduced the headroom of reference branch 10 by a threshold voltage. For integrated circuits having lower power supply levels, such as 1.8 v, this reduction in available headroom becomes a nontrivial effect.
Based upon the foregoing, there is a need for a current mirror having reduced nonlinear distortion at high frequency operation.
Embodiments of the present invention overcome the above-identified shortcomings and satisfy a significant need for a current mirror having reduced nonlinear distortion at relatively high frequencies. Nonlinear distortions are reduced in part by employment of a current amplifier with the input coupled through a capacitor to the input of the current mirror. An output of the current amplifier is either coupled to a node in the reference branch or a node in the mirror branch of the current mirror. The current amplifier may be a noninverting amplifier (when the output thereof is coupled to the reference branch) or an inverting amplifier (when the output thereof is coupled to the mirror branch). The current amplifier serves to restore the shape of the current mirror output signal, thereby reducing the nonlinear distortion of the current mirror.
Another embodiment of the present invention is adapted for use in applications that utilize multiple current mirrors, such as in a design in which two current mirrors are employed to provide a differential current signal. In this embodiment, the output of the current amplifier of a first current mirror of a pair of current mirrors is coupled to the second current mirror of the current mirror pair, and the output of the current amplifier of the second current mirror is coupled to the first current mirror.